Field
Exemplary embodiments of the present invention relate to a light emitting diode (LED) lighting apparatus with improved flicker index. More particularly, exemplary embodiments of the present invention relate to a sequential driving type alternating current (AC) LED lighting apparatus with improved flicker index, which can reduce light output deviation in an operation section by controlling an LED drive current supplied to LEDs based on the number of LEDs in a preset LED group.
Discussion of the Background
LEDs are generally operated by direct current (DC) driving. DC driving requires an AC-DC power converter such as an SMPS and the like, and such a power converter may cause an increase in manufacturing costs of luminaires, difficulty in reducing the size of the luminaires, deterioration in energy efficiency of the luminaires, and reduction in lifespan of the lighting apparatus due to short lifespan of the power converter.
To resolve such problems of DC driving, AC driving of LEDs has been suggested. However, an AC driving circuit may cause a reduction in power factor due to mismatch between input voltage and output power from the LEDs, and also severe flickering perceived by a user in the case where a non-light emitting section of LEDs is extended.
FIG. 1 is a conceptual view illustrating a flicker index. A definition and regulation of the flicker index as a reference flicker level in accordance with the Energy Star specifications will be described hereinafter.
(1) Definition of Flicker
Flicker means a phenomenon where lighting brightness is changed for a certain period of time, and severe flicker can be perceived as shaking light by a user. Flicker may be generated due to a difference between a maximum light output and a minimum light output for a certain period of time.
(2) Types of Flicker Index
(a) Flicker index: as shown in FIG. 1, the flicker index means a value obtained by dividing an area (Area1) above the level of average light output by the total light output area (Area1+Area2) on a light output waveform of one cycle. Thus, the flicker index is a value numerically indicating frequency of illumination above the level of average light output in one cycle and a lower flicker index indicates a better flicker level. “Improved flicker index” may mean a lower flicker index, that is, less flicker.
(b) Percent flicker or Modulation depth: percent flicker refers to a value numerically indicating a minimum intensity of light and a maximum intensity of light for a certain period of time. Such a percent flicker can be calculated according to the equation:100*(maximum intensity of light−minimum intensity of light)/(maximum intensity of light+minimum intensity of light).
(3) Flicker Level in Accordance with Energy Star Specifications                Light output waveform≧120 Hz        Flicker index≦frequency×0.001 (at Max. Dimmer, excluding flicker index at 800 Hz or more) (Thus, flicker index at 120 Hz≦0.12)        
As described above, the issue of flicker level is of increasing concern in performance of LED lighting apparatuses.
FIG. 2 is a schematic block diagram of a conventional four-stage sequential driving type LED lighting apparatus, and FIG. 3 is a waveform diagram depicting a relationship between drive voltage and LED drive current of the conventional four-stage sequential driving type LED lighting apparatus shown in FIG. 2. Next, referring to FIG. 2 and FIG. 3, problems of the conventional LED lighting apparatus will be described.
First, as shown in FIG. 2, a conventional LED lighting apparatus 100 may include a rectification unit 10, an LED light emitting unit 20, and an LED drive controller 30.
In the conventional LED lighting apparatus 100, the rectification unit 10 generates rectified voltage Vrec through rectification of AC voltage supplied from an external power source, and outputs the rectified voltage Vrec to the LED light emitting unit 20 and the LED drive controller 30. A full-wave rectification circuit, a half-wave rectification circuit, and the like may be used as the rectification unit 10. In FIG. 2, a bridge full-wave rectification circuit composed of four diodes D1, D2, D3, and D4 is shown. In addition, the LED light emitting unit 20 is composed of four LED groups including first to fourth LED groups 21 to 24, which may be sequentially turned on or off under control of the LED drive controller 30. The conventional LED drive controller 30 is configured to control the first to fourth LED groups 21 to 24 to be sequentially turned on or off according to a voltage level of the rectified voltage Vrec.
The conventional LED drive controller 30 increases or decreases the LED drive current according to a voltage level of input voltage (that is, rectified voltage (Vrec)) to perform constant current control in each of sequential driving sections. As a result, the LED drive current has a stepped waveform approaching a sine wave, whereby power factor (PF) and total harmonic distortion (THD) of the LED lighting apparatus are enhanced, thereby improving power quality of the LED lighting apparatus.
Operation of the conventional LED lighting apparatus 100 will be described in more detail with reference to FIG. 3. Referring to FIG. 3, in an operation section in which the rectified voltage Vrec is greater than or equal to a first forward voltage level Vf1 and less than a second forward voltage level Vf2 (a first stage operation section), the conventional LED drive controller 30 performs constant current control such that only the first LED group 21 is turned on and an LED drive current ILED becomes a first LED drive current ILED1. Similarly, in an operation section in which the rectified voltage Vrec is greater than or equal to the second forward voltage level Vf2 and less than a third forward voltage level Vf3 (a second stage operation section), the conventional LED drive controller 30 performs constant current control such that only the first LED group 21 and the second LED group 22 are turned on and the LED drive current ILED becomes a second LED drive current ILED2. Further, in an operation section in which the rectified voltage Vrec is greater than or equal to the third forward voltage level Vf3 and less than a fourth forward voltage level Vf4 (a third stage operation section), the conventional LED drive controller 30 performs constant current control such that the first to third LED groups 21 to 23 are turned on and the LED drive current ILED becomes a third LED drive current ILED3. Last, in an operation section in which the rectified voltage Vrec is greater than or equal to the fourth forward voltage level Vf4 (a fourth stage operation section), the conventional LED drive controller 30 performs constant current control such that all of the first to fourth LED groups 21 to 24 are turned on and the LED drive current ILED becomes a fourth LED drive current ILED4.
As shown in FIG. 3, the LED lighting apparatus is controlled such that the LED drive current in the first stage operation section (that is, the first LED drive current ILED1 is larger than the LED drive current in the second stage operation section (that is, the second LED drive current ILED2). Likewise, the LED lighting apparatus is controlled such that the third LED drive current ILED3 is larger than the second LED drive current ILED2 and the fourth LED drive current ILED4 becomes the largest drive current. Accordingly, the entire light output of the conventional LED lighting apparatus 100 has a stepped waveform, as shown in FIG. 3. Accordingly, since the total number and drive current of LEDs operated to emit light differ according to the operation sections, the conventional LED lighting apparatus 100 provides different light output according to the operation sections, and thereby may cause a difference in light output according to the operation sections, and severe flicker, as described above.
Further, the conventional LED lighting apparatus 100 is configured to control sequential driving based on drive voltage supplied to the LED light emitting unit 20, that is, based on the voltage level of the rectified voltage Vrec. However, such a voltage detection type may not satisfactorily reflect current/voltage characteristics based on temperature of LEDs. That is, regardless of different forward voltages of the LED groups according to operation temperatures of LEDs, the voltage detection type may not satisfactorily reflect IN characteristics depending upon the actual temperatures of the LEDs. As a result, the LED drive current (LED light output) may be instantaneously dropped or overshot at a time point that the operation section is changed, for example, at a time point of changing the operation section from the first stage operation section to the second stage operation section, thereby causing uneven light output of the LED lighting apparatus 100.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form any part of the prior art nor what the prior art may suggest to a person of ordinary skill in the art.